Here's a video taken from the inside of an E60 M5 on the German autobahn which happens across a BMW F10 M5 prototype doing some highway testing.
The 2005 E60 M5 blows past the F10 at more than 290 km/h (180 mph), as we can see on the speedometer reading in the video.
This is all legal, of course, having taken place on a speed limit-free stretch of highway in Germany.
The man was on holiday when he spotted the F10 M5 prototype and he estimates its speed at over 250 km/h (155 mph) and likely testing without a V-max limiter.
